1. Technical Field
The present invention relates generally to calibration of magnetic sensors.
2. Description of the Related Art
It is known to use magnetic sensors for making devices designed to determine an orientation and/or a position of a body, such as, for example, compasses and navigation systems. Magnetic sensors can determine the orientation of their own detection axes with respect to the Earth's magnetic field, which defines a fixed reference with respect to the ground.
Prior to their use, magnetic sensors must be calibrated so as to eliminate or at least reduce any imprecision and disturbance that can derive from different sources, such as, for example, process dispersions during manufacture, magnetic interference caused by the circuitry that controls the magnetic sensor, interference due to external causes (loudspeakers, batteries, ferromagnetic elements), dependence upon temperature and time. Calibration generally consists in selecting an appropriate set of gain and offset values for each detection axis of the magnetometer.
The calibration methods currently used are as a rule carried out just once at the moment of manufacture or else at the moment of installation of the device in an apparatus in which it is to be used (for example, in the navigation system of an automobile). According to known calibration methods, a magnetometer is rotated through 360° in a horizontal plane so as to determine the direction of the magnetic North. In the case of triaxial sensors, in a second step the magnetic sensor is rotated also in a vertical plane passing through the magnetic North. The gain and offset values are then modified until a response of the magnetic sensor is obtained (in particular minimum and maximum values of intensity) in line with a magnetic field value expected in the point where the calibration is carried out. The methods of this type, however, present the disadvantage of not being repeatable after installation of the magnetic sensor because it is necessary to restore well-defined environmental conditions and carry out accurate rotations of the magnetic sensor according to degrees of freedom that are often no longer available. Furthermore, calibration as described necessarily requires the comparison with a value of magnetic field supplied by an already calibrated reference, which is not easily available. Consequently, albeit precise and accurate they may be, known calibration methods are not useful for compensating for the effects of sources of disturbance and error that intervene after the magnetic sensor has been set in operation.
Other known methods are based on statistical processing of sets of measurements made during a random movement of the magnetic sensor. In practice, the average of the measurements according to each detection axis is estimated and used as indication of the offset values of the magnetic sensor. In this case, the method is irrespective of the availability of a previously calibrated reference, but does not enable calibration of the gain values.